Fig. 16.16 ). At slow heart rates, the effective refractory period of the Purkinje fibers is especially prolonged.
In contrast to Purkinje fibers, the effective refractory period of AV node cells does not change appreciably over the normal range of heart rates and actually increases at very rapid heart rates. Therefore, when the atrium is excited at high repetition rates, it is the AV node that normally protects the ventricles from these excessively high frequencies.
The first portions of the ventricles to be excited by impulses arriving from the AV node are the interventricular septum (except the basal portion) and the papillary muscles. The activation wave spreads into the substance of the septum from both its left and right endocardial surfaces. Early contraction of the septum makes it more rigid and allows it to serve as an anchor point for contraction of the remaining ventricular myocardium. Furthermore, early contraction of the papillary muscles prevents eversion of the AV valves into the atria during ventricular systole.
The endocardial surfaces of both ventricles are activated rapidly, but the wave of excitation spreads from endocardium to epicardium at a slower velocity (≈0.3 to 0.4 m/sec). The epicardial surface of the right ventricle is activated earlier than that of the left ventricle because the right ventricular wall is appreciably thinner than the left. In addition, the apical and central epicardial regions of both ventricles are activated somewhat earlier than their respective basal regions. The last portions of the ventricles to be excited are the posterior basal epicardial regions and a small zone in the basal portion of the interventricular septum.
The conditions necessary for reentry (defined in the following box) are illustrated in
Fig. 16.23 . In each of the four panels, a single bundle of cardiac fibers splits into a left branch and a right branch. A connecting bundle runs cSimilar directional changes in the refractory period also occur in ventricular myocytes in response to changes in heart rate.
•Fig. 16.23 The Role of Unidirectional Block in Reentry. A, Anexcitationwavetravelingdownasinglebundle(S)offiberscontinuesdowntheleft(L)andright(R)branches.Thedepolarizationwaveenterstheconnectingbranch(C)frombothendsandisextinguishedatthezoneofcollision.B, ThewaveisblockedintheLandRbranches.C, AbidirectionalblockexistsintheRbranch.D, AunidirectionalblockexistsintheRbranch.Theantegradeimpulseisblocked,buttheretrogradeimpulseisconductedthroughandreenterstheSbundle.
between the two branches. Normally, the impulse moving down the single bundle is conducted along the left and right branches (see
Fig. 16.23A ). As the impulse reaches the link to the connecting bundle (
Fig. 16.23C), it enters from both sides and becomes extinguished at the point of collision. The impulse from the left side cannot proceed because the tissue beyond is absolutely refractory; it has just been depolarized from the other direction. The impulse also cannot pass through the connecting bundle from the right for the same reason.
Fig. 16.23B shows that the impulse cannot complete the circuit if an antegrade block exists in the left and right branches of the fiber bundle. Also, if a bidirectional block exists at any point in the loop (e.g., branch R in
Fig.
16.23C ), the impulse also cannot reenter.
Incertainconditions,acardiacimpulsemayreexcitesomemyocardialregionthroughwhichithadpassedpreviously.Thisphenomenon,knownasreentry, isresponsibleformanyclinicalarrhythmias(disturbancesincardiacrhythm).Thereentrymaybeorderedorrandom.Inorderedreentry,theimpulsetraversesafixedanatomicalpath,whereasinrandomreentry,thepathcontinuestochange.
A necessary condition for reentry is that at some point in the loop the impulse can pass in one direction but not in the other. This phenomenon is called unidirectional block. As shown in